1. Field of the Invention
The present invention relates to a projection lens unit. More particularly, it relates to a method and device for maintaining the proper focus of a projection lens unit in relatively high temperature operating conditions.
2. Description of the Art
Projection television systems are well known. In general, such a system takes a picture from a relatively small CRT (cathode ray tube) screen and projects and magnifies it through a projection lens onto a larger screen. FIG. 2 illustrates a projection system in which a picture is projected from a video projector 16 onto a screen 17 placed a predetermined distance in front of the projector 16.
Optical coupling is a technique used in projection systems to improve the clearness and contrast of the projected image. In optical coupling, a filling liquid is placed in the optical path of the CRT's faceplate. The filling liquid is one that is highly transparent and has approximately the same refractive index as the faceplate. In general, the filling liquid decreases interfacial reflection by reflecting undesired light.
FIG. 1 illustrates an axial cross section of a known projection lens unit P. The unit P includes a CRT 3 mounted on a right side of an "optical coupling" (OC) housing 2. An OC lens 4 is mounted on a left side of the OC housing 2. The OC lens 4 is located to the right of a projection lens 1.
The OC housing 2 includes gutters 5 and 6. Sealing rubbers 7 and 8 are inserted in the gutters 5 and 6, respectively. The OC lens 4 is mounted on a left side of the OC housing 2, and the CRT 3 is mounted on a right side of the OC housing 2. The sealing rubbers 7 and 8 are tightly pressed to a CRT faceplate 18 to form a liquid tight seal.
The OC housing 2 includes a pressure adjusting hole 9. A pressure loosening diaphragm 10, made of elastic material such as rubber, is mounted over the pressure adjusting hole 9. The pressure loosening diaphragm 10 has around its rim a pressure sealing part 11, which is seated in a sealing gutter formed around the pressure adjusting aperture 9. A holding cover 12 is mounted over the pressure adjusting hole 9 and diaphragm 10. The holding cover 12 presses the pressure sealing part 11 against the OC housing 2, thereby providing a liquid tight seal over the hole 9. This liquid tight seal also seals an OC chamber 14 formed by the OC housing 2, the CRT 3, and the OC lens 4. The holding cover 12 includes a ventilation hole 13 which exhausts air when the pressure loosening diaphragm 10 moves upward.
Referring still to FIG. 1, prior to sealing the cover 12 in place, the OC chamber 14 is filled with an OC liquid 15 through the pressure adjusting aperture 9. Suitable OC liquids 15 include ethylene glycol, ethylene glycol mixed with a small amount of pure water, ethylene glycol mixed with glycerin, and ethylene glycol mixed with glycerin and a small amount of pure water.
When the CRT 3 is turned on, the picture displayed on the CRT screen is projected and magnified by the projection lens 1 onto a screen 17 located in front of the video projector 16 as shown in FIG. 2. The components of the projection system must be precisely aligned so that the image projected onto the large screen 17 remains focussed.
One of the problems with optical coupling projection systems is that high temperature operating conditions can cause the projected picture to become unfocussed. This is because heat changes the refractive index of the liquid, thereby changing the focus of the projected image.
Because the CRT of a video projector requires a large amount of light in order to perform the required magnification and projection, video projectors are driven with a large amount of electric power compared with the power requirements of an ordinary CRT being used for direct viewing. Therefore, the temperature of the CRT itself, particularly its faceplate, can rise over 100.degree. C. during use.
The OC liquid 15 functions as a cooling liquid for lowering the temperature of the faceplate 18. Heat is transmitted to the OC housing 2 through the OC liquid 15 and dissipated into the air. Increased temperature causes the OC liquid 15 to expand, thereby pushing upwards on the pressure loosening diaphragm 10. Increased temperature also decreases the refractive index of the OC liquid 15 in proportion to the amount of volume expansion. In the case of ethylene glycol, for example, the refractive index of E-ray (545 nano meter) is 1.4308 at 20.degree. C. and 1.4146 at 70.degree. C. The OC liquid temperature gradually rises while the CRT 3 is being operated, and, in a typical video projector, the liquid temperature can reach as high as 70.degree. C. to 80.degree. C. Noticeable defocussing occurs when the OC liquid 15 reaches about 20.degree. C.
Accordingly, there is a need for a projection lens unit that takes advantage of the benefits of an OC system but avoids the drawbacks associated with the defocussing that occurs at high temperatures. To date, no positive countermeasures have been taken against the drawbacks described above.